1. Field of the Invention
The present invention is directed to a valve control apparatus for controlling a gas control valve to deliver a precisely measured flow of gas, and more particularly, to a valve control apparatus which controls a valve to precisely obtain a particular rate of gas flow or maintain the flow within a particular range, by controlling the valve to open or close in accordance with a measured rate of flow to obtain a desired rate of gas flow.
2. Description of the Related Art
It is often necessary to control or limit a flow of gas quickly and precisely. For example, a gas flow line can have a flowmeter for monitoring gas flow. The flowmeter is a delicate and expensive device. If the gas flow in the gas flow line exceeds a certain rate (or delivery), the flowmeter will be damaged or destroyed.
For example, in an Air Inleakage Monitor (AIM), produced by Westinghouse Electric Corp., a flowmeter is used to measure the leakage of air into a steam turbine. As shown in FIG. 1, air is routed from the pipe 2 through a flowmeter 4 via the by-pass 6 as long as the flow is below 50 cfm. However, when air flow in the by-pass reaches 50 cfm, a bladder 8 is deflated by a valve control unit 10 so that the air flows through the pipe 2, thus significantly reducing air flow through the by-pass 6. The by-pass 6 is connected to the flowmeter 4 at a pipe joint 11 and to an extension of pipe 2 at pipe 12. During normal operation of the turbine, deflation of the bladder is sufficient to prevent air flow through the flowmeter from exceeding 50 cfm, so that damage to the flowmeter is avoided and measurement of air flow by the flowmeter can continue, although less accurately. However, there are certain circumstances in which net air flow through the air inleakage monitor is considerably higher than in normal operation. For example, during pump-down the net flow can reach and exceed 6000 cfm. Under these circumstances, the air flow through the flowmeter can exceed 50 cfm even when the bladder is deflated. Thus, the bladder device offers no protection against damage or destruction to the flowmeter.